Gabriele Gerlach

The behaviour and ecology of the zebrafish, Danio rerio.

The zebrafish Danio rerio, is an important model organism in developmental genetics, neurophysiology and biomedicine, but little is known about its natural ecology and behaviour. It is a small, shoaling cyprinid, native to the flood‐plains of the Indian subcontinent, where it is found in shallow, slow‐flowing waters. Zebrafish are group spawners and egg scatterers, although females are choosy with respect to sites for oviposition and males defend territories around such sites. Laboratory studies of zebrafish behaviour have encompassed shoaling, foraging, reproduction, sensory perception and learning. These studies are reviewed in relation to the suitability of the zebrafish as a model for studies on cognition and learning, development, behavioural and evolutionary ecology, and behavioural genetics.

Smelling home can prevent dispersal of reef fish larvae

Many marine fish and invertebrates show a dual life history where settled adults produce dispersing larvae. The planktonic nature of the early larval stages suggests a passive dispersal model where ocean currents would quickly cause panmixis over large spatial scales and prevent isolation of populations, a prerequisite for speciation. However, high biodiversity and species abundance in coral reefs contradict this panmixis hypothesis. Although ocean currents are a major force in larval dispersal, recent studies show far greater retention than predicted by advection models. We investigated the role of animal behavior in retention and homing of coral reef fish larvae resulting in two important discoveries: (i) Settling larvae are capable of olfactory discrimination and prefer the odor of their home reef, thereby demonstrating to us that nearby reefs smell different. (ii) Whereas one species showed panmixis as predicted from our advection model, another species showed significant genetic population substructure suggestive of strong homing. Thus, the smell of reefs could allow larvae to choose currents that return them to reefs in general and natal reefs in particular. As a consequence, reef populations can develop genetic differences that might lead to reproductive isolation.

Calculations of population differentiation based on GST and D: forget GST but not all of statistics!

G ST‐values and its relatives (FST) belong to the most used parameters to define genetic differences between populations. Originally, they were developed for allozymes with very low number of alleles. Using highly polymorphic microsatellite markers it was often puzzling that GST‐values were very low but statistically significant. In their papers, Jost (2008) and Hedrick (2005) explained that GST‐values do not show genetic differentiation, and Jost suggested calculating D‐values instead. Theoretical mathematical considerations are often difficult to follow; therefore, we chose an applied approach comparing two artificial populations with different number of alleles at equal frequencies and known genetic divergence. Our results show that even for more than one allele per population GST‐values do not calculate population differentiation correctly; in contrast, D‐values do reflect the genetic differentiation indicating that data based on GST‐values need to be re‐evaluated. In our approach, statistical evaluations remained similar. We provide information about the impact of different sample sizes on D‐values in relation to number of alleles and genetic divergence.

Kin recognition and inbreeding avoidance in zebrafish, Danio rerio, is based on phenotype matching

Differentiating kin from nonkin enables organisms of many species to allocate resources or altruistic behaviour towards related conspecifics and to avoid mating with close relatives. Kin recognition mechanisms can vary among species and may reflect the social environment. Learned familiarity with nest or shoalmates may serve as a good indication that individuals are related, as long as the social system is sufficiently stable to avoid intermingling of unrelated individuals with siblings. Phenotype matching allows for recognition of even unfamiliar kin because individuals establish an olfactory, visual or acoustic template for their kin during early development and compare this template to cues from unfamiliar individuals later in life. We tested which kin recognition mechanism is used by zebrafish and we found that the preference for kin changes with sexual maturity. The olfactory preference of laboratory-bred juveniles and reproductively active adults were tested in an odour choice flume. Juveniles of mixed-sex groups spent more time on the side of unfamiliar kin than unfamiliar nonkin, indicating that kin recognition and preference are based on a phenotype matching mechanism. They also preferred familiar kin to unfamiliar kin, showing that familiarity with individuals enhances kin preference. Preference changed with sexual maturity. Adult females preferred the odour of unfamiliar, unrelated males to unfamiliar brothers, indicating inbreeding avoidance. Adult male zebrafish showed no preference for the odour of related or unrelated females. To our knowledge, this is the first study testing different kin recognition mechanisms and switching of kin preference in the same species of fish.

Kin-structured subpopulations in Eurasian perch ( Perca fluviatilis L.)

Based on ecological and behavioural studies it has been assumed that Eurasian perch (Perca fluviatilis) within one lake may not represent one panmictic population, but that they are subdivided into subpopulations. In order to investigate the genetic substructuring of populations, we used gene frequencies of five microsatellite loci to compare perch from six different sites from Lake Constance, Germany, and as outgroups perch from the lake Grosser Vätersee, Berlin, and two Swiss lakes, Lake Zurich and Lake Walensee. We examined whether homing behaviour of subadults to the spawning sites of their parents occurs and whether philopatric behaviour of adults results in significant population genetic substructuring. The distribution of genetic variation revealed two major, genetically distinct populations in Lake Constance: one in the eastern part of the lake and another in the western part (GST=0.07). Within each of these two populations, no further genetic substructuring, nor any indication of inbreeding could be detected, either because genetic exchange was sufficiently high or because the time since separation has been too short. Homing behaviour of subadults to parental spawning sites after having spent several weeks of their life cycle in the pelagic zone could not be detected. Instead, subadults stay within either the western or the eastern region of the lake. There is evidence that some shoals contain full- and half-sibs. Despite females spawning in close proximity to each other, some siblings stay together. This might suggest that perch possess kin preferences and kin recognition.

Kin recognition in zebrafish: a 24-hour window for olfactory imprinting.

Distinguishing kin from non-kin profoundly impacts the evolution of social behaviour. Individuals able to assess the genetic relatedness of conspecifics can preferentially allocate resources towards related individuals and avoid inbreeding. We have addressed the question of how animals acquire the ability to recognize kin by studying the development of olfactory kin preference in zebrafish (Danio rerio). Previously, we showed that zebrafish use an olfactory template to recognize even unfamiliar kin through phenotype matching. Here, we show for the first time that this phenotype matching is based on a learned olfactory imprinting process in which exposure to kin individuals on day 6 post fertilization (pf) is necessary and sufficient for imprinting. Larvae that were exposed to kin before or after but not on day 6 pf did not recognize kin. Larvae isolated from all contact with conspecifics did not imprint on their own chemical cues; therefore, we see no evidence for kin recognition through self-matching in this species. Surprisingly, exposure to non-kin odour during the sensitive phase of development did not result in imprinting on the odour cues of unrelated individuals, suggesting a genetic predisposition to kin odour. Urine-born peptides expressed by genes of the immune system (MHC) are important messengers carrying information about ‘self’ and ‘other’. We suggest that phenotype matching is acquired through a time-sensitive learning process that, in zebrafish, includes a genetic predisposition potentially involving MHC genes expressed in the olfactory receptor neurons.

Assessing hidden species diversity in the coral Pocillopora damicornis from Eastern Australia

The incredible range of morphological plasticity present in scleractinian corals has confused the taxonomy of the group, prompting the introduction of “ecomorphs” to explain the observed correlation between local environmental conditions and phenotypic variation. Pocilloporadamicornis (Linnaeus, 1758) represents one of the best known examples of eco-phenotypic variation in scleractinian corals with a variety of forms and reproductive strategies reported across its global distribution range. Here, we reconstruct genealogical relationships of P. damicornis colonies collected from thirteen locations along the East Australian coast to examine the relationship between genetic and phenotypic diversity in this species. Haplotype networks computed from two mitochondrial DNA regions (CR, ORF) indicate that the range of morphotypes observed within this taxon fall into at least five genetically distinct mitochondrial lineages. Nuclear (HSP70, ITS2) haplowebs on the other hand recover sharp genetic discontinuities among three of the morphological groups. We conclude that P. damicornis from Eastern Australia constitutes a cryptic species complex. The misinterpretation of taxonomical units within P. damicornis may well explain its perceived variation in the ecology, biology and life history across its range.

Pheromonal regulation of reproductive success in female zebrafish: female suppression and male enhancement

Competition among females represents an important but rather neglected variable in studies of sexual selection. Probably to avoid injury to self and offspring, competition among females is often displayed without much physical interaction and therefore remains harder to observe. Here, we show for the first time in a teleost fish, the zebrafish Danio rerio, that females can use waterborne pheromones to suppress reproduction by other females. Female zebrafish that had been exposed to another females pheromones for 4 days prior to mating spawned significantly fewer viable eggs than females held in isolation or exposed to male pheromones. Male pheromones not only stimulated female reproduction but also increased the quality and viability of eggs. In grouped females, reproductive success correlated with dominance rank. These results indicate that fish pheromones function to control mates and competitors in addition to serving as reproductive timing signals. Differences in female reproductive success observed in many fish species might be explained by this mechanism.

Emigration mechanisms in feral house mice – a laboratory investigation of the influence of social structure, population density, and aggression

Abstract Emigration in small mammals may be strongly related to social factors, but direct observations of emigrants are rare. Feral house mice (Mus domesticus) were studied using a population cage system that allowed continuous observation of individually marked animals. Mice that left their natal cage and took up residence in cages that could only be reached by crossing a water barrier were defined as emigrants. Six pairs of house mice with their litters were placed in the system, and data on aggressive interactions, body weight, reproduction, mortality and emigration were collected daily. Both sexes emigrated, but males did so twice as often as females. Population density was not correlated with the frequency of aggression, and had no effect on the weight of emigrating individuals. Male emigrants suffered more aggression before emigration than their non-emigrant brothers of the same age; they were aggressively driven out by other males, predominantly by the father. Female emigration depended on the female’s chances of reproduction. The probability of a female reproducing decreased with increasing birth order. Females born in a late litter, who therefore had only a low chance of reproduction, dispersed earlier than those of early litters. Resident males were reproductively suppressed. Male offspring had two different strategies for attaining top rank. They could develop rapidly and reach sexual maturity early on, but face competition with the father, risking being forced to emigrate. Alternatively, they could develop slowly, stay within their family and wait for a chance to take over the dominant position. It is concluded that emigration in male and female feral house mice is caused by intrasexual competition.

Kin and population recognition in sympatric Lake Constance perch (Perca fluviatilis L.) : can assortative shoaling drive population divergence?

Prior studies have shown that perch (Perca fluviatilis L.) of Lake Constance belong to two genetically different but sympatric populations and that local aggregations of juveniles and adults contain closely related kin. In this study, we analysed the genetic structure of pelagic perch larvae to investigate if kin-structured shoals already exist during early ontogenetic development or might be the result of homing to natal sites. Analysis of the gene frequencies at five microsatellite loci revealed that three out of five pelagic aggregations of larvae showed significant accumulation of kin. To investigate possible mechanisms of shoal formation, we tested if perch use olfactory cues to recognize their kin. Choice tests in a fluviarium showed preference for odours of unfamiliar kin vs unfamiliar non-kin. Additionally, we showed that perch could differentiate between the odours of the two sympatric populations and significantly preferred unfamiliar and unrelated conspecifics of their own over the foreign population. Our results present a behavioural mechanism that can lead to the observed formation of kin-structured shoals in perch. We further discuss if the ability to discriminate between their own and a foreign population can result in assortative mating within populations and thus form the basis of “socially mediated speciation” in perch.